Not Applicable.
The present embodiments relate to wireless communications systems and are more particularly directed to selecting paths for further processing in such systems.
Wireless communications have become very prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. One such advancement includes the use of spread spectrum communications, including that of code division multiple access (xe2x80x9cCDMAxe2x80x9d). In such communications, a user station (e.g., a hand held cellular phone) communicates with a base station, where typically the base station corresponds to a xe2x80x9ccell.xe2x80x9d More particularly, CDMA systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected user station within the cell to determine the proper recipient of a data signal.
CDMA continues to advance along with corresponding standards that have brought forth a next generation wideband CDMA (xe2x80x9cWCDMAxe2x80x9d). WCDMA includes alternative methods of data transfer, one being time division duplex (xe2x80x9cTDDxe2x80x9d) and another being frequency division duplex (xe2x80x9cFDDxe2x80x9d). The present embodiments may be incorporated in either TDD or FDD and, thus, both are further introduced here. TDD data are transmitted in one of various different forms, such as quadrature phase shift keyed (xe2x80x9cQPSKxe2x80x9d) symbols or other higher-ordered modulation schemes such as quadrature amplitude modulation (xe2x80x9cQAMxe2x80x9d) or 8 phase shift keying (xe2x80x9cPSKxe2x80x9d). In any event, the symbols are transmitted in data packets of a predetermined duration or time slot. Within a data frame having 15 of these slots, bi-directional communications are permitted, that is, one or more of the slots may correspond to communications from a base station to a user station while other slots in the same frame may correspond to communications from a user station to a base station. Further, the spreading factor used for TDD is relatively small, whereas FDD may use either a large or small spreading factor. FDD data are comparable in many respects to TDD including the use of 15-slot frames, although FDD permits a different frequency band for uplink communications (i.e., user to base station) versus downlink communications (i.e., base to user station), whereas TDD uses a single frequency in both directions.
Due to various factors including the fact that CDMA communications are along a wireless medium, an originally transmitted communication from a base station to a user station may arrive at the user station at multiple and different times. Each different arriving signal that is based on the same original communication is said to have a diversity with respect to other arriving signals originating from the same transmitted communication. Further, various diversity types may occur in CDMA communications, and the CDMA art strives to ultimately receive and identify the originally transmitted data by exploiting the effects on each signal that are caused by the one or more diversities affecting the signal.
One type of CDMA diversity occurs because a transmitted signal from a base station is reflected by objects such as the ground, mountains, buildings, and other things that it contacts. As a result, a same single transmitted communication may arrive at a receiving user station at numerous different times, and assuming that each such arrival is sufficiently separated in time, then each different arriving signal is said to travel along a different channel and arrive as a different xe2x80x9cpath.xe2x80x9d These multiple signals are referred to in the art as multiple paths or multipaths. Several multipaths may eventually arrive at the user station and the channel traveled by each may cause each path to have a different phase, amplitude, and signal-to-noise ratio (xe2x80x9cSNRxe2x80x9d). Accordingly, for one communication from one base station to one user station, each multipath is originally a replica of the same originally transmitted data, and each path is said to have time diversity relative to other multipath(s) due to the difference in arrival time which causes different (uncorrelated) fading/noise characteristics for each multipath. Although multipaths carry the same user data to the receiver, they may be separately recognized by the receiver based on the timing of arrival of each multipath. More particularly, CDMA communications are modulated using a spreading code which consists of a series of binary pulses, and this code runs at a higher rate than the symbol data rate and determines the actual transmission bandwidth. In the current industry, each piece of CDMA signal transmitted according to this code is said to be a xe2x80x9cchip,xe2x80x9d where each chip corresponds to an element in the CDMA code. Thus, the chip frequency defines the rate of the CDMA code. Given the transmission of the CDMA signal using chips, then multipaths separated in time are distinguishable at the receiver because of the low auto-correlations of CDMA codes. Also, given that numerous multipaths may arrive at a receiving user station, the prior art endeavors to select certain of these multipaths and then to perform various processing on those paths in an effort to combine the signals to remove the effects of the diversity and to better recover the originally-transmitted data represented by those signals. However, before this selection process occurs, various acquisition steps are performed by the receiving user station and which are discussed below by way of further introduction.
According to the prior art a receiving user station first processes incoming signals, often using what is referred to as a searcher and in a first acquisition stag. Specifically, each incoming frame includes a so-called synchronization channel against which correlations may be made by the receiving user station for purposes of acquisition, where the synchronization channel includes two codes, namely, a ar my synchronization code (xe2x80x9cPSCxe2x80x9d) and a secondary synchronization code (xe2x80x9cSSCxe2x80x9d). The PSC is presently a 256 chip Golay code and the same PSC code is transmitted from numerous base stations. Each base station group transmits a unique set of SSC code words. In any event, during the first acquisition stage, the user station continuously samples information in at east one slot and performs a PSC correlation on those samples. For example, this technique may be implemented by applying the received information to a matched filter having the 256 chip Golay code of the PSC as coefficients to the filter, and the results of the correlations may be processed further such as through the use of averaging. Moreover, the number of measured correlations typically depends on the data rate and sample rate. For example, presently a single slot in a frame has a 667 xcexcsec duration corresponding to a chip rate of 3.84 Mcps (although in the past the chip rate was 4.096 Mcps and provided a 625 xcexcsec slot duration). Further, such a slot typically includes 2560 chips, and the PSC correlation measurement or sampling is typically twice per chip, thereby giving rise to a total of 5120 sample positions evaluated per slot. In any event, as a result of these measurements, one or more paths within the evaluated time period are found to have relatively large PSC correlations, and the position(s) of these path(s) are generally used to identify the timing of incoming frames. Lastly, since the PSC is the same for various base stations, then note that the identified one or more paths may correspond to one or more base stations.
Also according to the prior art, a receiving user station next processes incoming signals to perform a second acquisition stage with respect to the SSCs in the incoming signals. Recalling that the SSC is base-station specific, note that the second acquisition stage is therefore directed to a particular base station. Further, note that the accumulation of several SSCs across a frame or frames are sometimes referred to in the art as comma free codes. Thus, correlations of the comma free codes are measured during the second acquisition stage. Once the SSCs are detected in the incoming signal, the user station is thereby informed of the data (i.e., frame) location within the base station communications. Further, once the user station has detected a unique base station SSC, the user station also may identify a so-called group of long codes that is also unique to, and transmitted by, the ase station, where that long code is then usable by the user station to demodulate data received from the base station.
Continuing with the prior art and as now illustrated with reference to FIG. 1, once the long code is determined by the user station, then the user station defines, relative to a time slot window 10, a search window 20, where typically search window 20 is a time period centered about a path P1. Typically, search window 20 is on the order of one-tenth the duration of time slot window 10. Thus, in the illustrated example wherein time slot window 10 is 667 xcexcsec, then search window 20 is 66.7 xcexcsec. Given the chip rates mentioned above, therefore, search window 20 includes 256 chips. Further, search window 20 is examined at a rate of two samples per chip, thereby giving rise to a total of 512 sample positions within search window 20 and spaced apart from one another in one-half chip increments. Having established search window 20, next the receiver performs a so-called delay profile estimation (xe2x80x9cDPExe2x80x9d), where this DPE is across every sample position in search window 20 (i.e., 512 positions). The result of this DPE is shown in FIG. 1 as a spectrum 30. Next, the receiver identifies a number of paths corresponding to the respective peak locations of spectrum 30, and those paths are then further processed so that the diversity may be exploited toward recovering the actual symbols from the various paths such as by combining those paths. Such an operation for large spreading factor FDD communications is often performed by assigning each identified path to a corresponding finger of a rake receiver, where the rake receiver performs what is referred to as a maximal ratio combining (xe2x80x9cMRCxe2x80x9d) operation which combines the various paths taking into account the respective delays of those paths. Similarly, for a small spreading factor FDD communication, or for a TDD communication, a comparable operation is performed by what may be referred to as a joint detector which includes both a rake receiver and an equalizer function to combine the various identified paths.
While the preceding prior art operations have proven useful in identifying multipaths for further processing, the present inventors have observed various drawbacks from this approach. Specifically, the DPE across search window 20 requires considerable computational complexity. For example, the DPE with respect to search window 20 is typically performed with respect to the pilot symbols at each sample position, and to perform this operation first the long code is ascertained in the second acquisition stage, and then for the DPE the long code must be stripped from each pilot symbol. As another example, this same operation is performed for every half-chip sample position within search window 20 and, thus, each calculation must be repeated 512 times. Thus, the various above-described details yield a very large number of computations to perform the DPE across search window 20. The large computational demands of the preceding have corresponding drawbacks. For example, such operations consume various resources such as hardware availability and power. These resources may be particularly valuable at the receiver which is often a portable device and, as such, where it is desired to reduce the complexity and power requirements of such a device.
In view of the preceding, there is a need to improve the identification and selection of multipaths for combining those selected signals such as in an MRC process, and this need is addressed by the preferred embodiments as described below.
In the preferred embodiment, there is a wireless receiver. The receiver comprises at least one antenna for receiving a plurality of frames in a form of a plurality of paths. Each of the plurality of frames comprises a plurality of time slots, and each of the plurality of time slots comprises a plurality of symbols. Further, each of the plurality of paths has a corresponding sample position, wherein the plurality of symbols comprise a primary synchronization code symbol. The receiver further comprises circuitry for correlating a primary synchronization code across a group of the plurality of symbols and circuitry for identifying a plurality of path positions within the group. Each of the identified plurality of path positions corresponds to a respective one of a plurality of largest-amplitude paths represented within the group as detected in response to the circuitry for correlating. The receiver further comprises circuitry for defining a plurality of sub-windows. Each of the plurality of sub-windows comprises a plurality of sample positions, and each of the plurality of sub-windows includes at least one of the plurality of identified path positions. Finally, the receiver further comprises circuitry for combining paths selected from the sample positions within the plurality of sub-windows. Other circuits, systems, and methods are also disclosed and claimed.